An optical fiber amplifier using an Erbium-doped fiber amplifier (EDFA) has a high saturation output, a high gain, and non-polarization dependency.
For these properties, such an EDFA is used for an optical booster amplifier, an optical repeater, an optical preamplifier, etc. Consequently, the freedom for optical communication systems is remarkably expanded. Optical communication systems in which such EDFAs are used have been described in reports entitled "A 212 km non-repeated transmission experiment at 1.8 Gb/s using LD pumped ER.sup.3+ -doped fiber amplifier in an IM/direct-detection repeater system" presented at OFC'3 89, PD-1, January 1989 by Hashimoto et al, "Impact applied on optical communication systems by Er-doped fiber optical amplifiers" presented at O plus E, 113, April, 1989 by Shimada, and "Trunk and distribution network application of Erbium-doped fiber amplifier" presented at IEEE, Journal of Lightwave Technology, Vol. 9, No. 2February 1991 by Nakagawa, et al.
Further, optical communication systems in which wavelength multiplexing light signal transmission is adopted, or fiber type mach-Zender gain equalizers are used has been described in a report entitled "A 100-channel optical FDM six-stage in-line amplifier system employing tunable gain equalizers" presented at IEEE, Photonixs Tech. Lett. Vol. 5, No. 2, February 1993 by Toba, et al.
In the optical communication system using the EDFAs, however, there is a disadvantage in that the deterioration of a light signal at wavelengths having low gains in large due to the dependency f signal lights on wavelengths in the EDFAS, when the wavelength multiplexing communication system is used. In order to overcome the disadvantage, a system in which a signal wavelength band of each signal light is set proportionally to a gain of each wavelength band in accordance with a gain-to-band property of an optical amplifier as has been proposed in the Japanese Patent Kokai No. 3-89644.
In a system using the Mach-Zender gain equalizer, there are disadvantages in that accumulated gain differences are difficult to be compensated for in carrying out step-by-step amplification, and the fabrication cost of the equalizer is high. In using a fiber type Mach-Zender gain controller, there is a disadvantage in that a control response speed is not satisfactory, because a wavelength property of a transmission factor is controlled by a fiber temperature. Even in analog signal light transmission, composite secondary distortion occurs to induce deterioration of transmssion signals due to the dependency of a gain in an EDFA on a signal light wavelength. For this disadvantage, a proposal in which a gain is corrected by using a band-pass filter having a tuning wavelength slightly deviated from a wavelength of a signal light has been described in a report of B-705 in the Institute of Electronics Information and Communications Engineerings, 1992 by Kikushima. However, the proposal cannot be applied to a system for transmitting signals lights of multiple-wavelength.